The present invention relates to a shape memory device that exhibits cyclical shape change, and the process for producing the same. In one embodiment, a multi-alloy NiTi thin film is deposited by DC sputtering.
NiTi is a shape memory alloy (SMA) that is capable of recovering strains on the order of 10%. This effect, referred to as the shape memory effect (SME), occurs when the material undergoes a phase transformation from the low temperature martensitic phase to the high temperature austenitic phase. In the martensitic phase the material is deformed by preferential alignment of twins. Unlike permanent deformations associated with dislocations, deformation due to twinning is fully recoverable when heated to the austenite phase.
A difficulty in using thin film SMA is that the deposited films exhibit the one way shape memory effect (SME) only. An SME material recovers its original shape after heating to the austenite phase but does not revert back to its deformed state when cooled. In order to achieve cyclic actuation, a biasing force such as a spring is necessary to deform the material when in the martensite phase. Implementing a bias force on thin film structures present significant manufacturing obstacles, an additional challenge for using thin film SME in MEMS actuators.
The first work to incorporate thin film NiTi in devices used a micro-machining process developed by Walker et al. in 1990 [J. A. Walker, K. J. Gabriel, and M. Mehregany, Sens. Actuators, Vols. A21-A23, p. 243, 1990]. Walker et al. used a wet chemical etchant (HF+HN03+H20) to pattern a free standing serpentine NiTi spring. The structures were curled when released and uncurled when heated, they attributed this to the shape memory effect. However, the films were amorphous as deposited. In 1990 Bush and Johnson at the TiNi Alloy Company showed the first definitive evidence of SME in NiTi films [J. D. Busch, A. D. Johnson, et. al., xe2x80x9cShape-memory properties in Nixe2x80x94Ti sputter deposited filmxe2x80x9d, J. Appl. Phy., Vol.68, p.6224, 1990]. Using a single target (50/50 atm % NiTi), with a DC magnetron sputtering system they pre-sputtered for 3 hours. Sputtering of the film was performed with a PAr=0.75 mTorr, V=450V, I=0.5A, and a target substrate distance of 2.25 inches was used. The as-deposited, film was shown by XRD to be amorphous and, after vacuum annealing at 550xc2x0 C. for 30 minutes, exhibited the SME although transformation temperatures were 100xc2x0 C. lower than the target material.
To achieve a cyclical, two-way effect, abiasing force is required to reshape the NiTi when cooled. Kuribayshi introduced a biasing force by tailoring precipitates in his films such that there were compressive and tensile stresses on opposite sides of his film [K. Kuribayashi, T. Taniguchi, M. Yositake, and S. Ogawa, xe2x80x9cMicron sized arm using reversible TiNi alloy tin film actuatorsxe2x80x9d. Mat. Res. Soc. Symp. Pro., vol.276, p.167, 1992]. The film curled when in the martensitic phase and when heated to the austenite phase flattened because the higher modulus overcomes the residual stresses. The fabrication process required complicated heat treatments. The stability of these precipitates can degrade over numerous thermal cycles.
Thin film TiNi actuators are well suited for MEMS devices because of their large work energy densities. However, the difficulties associated with depositing this material has limited its access by the MEMS community. To address this issue, researchers focused on deposition, heat treatments, and thermomechanical characterization of the film [J. D. Busch, M. H. Berkson, and A. D. Johnson, Phase transformations in sputtered NiTi film: effects of heat treatment and precipitates. Mat. Res. Soc. Symp. Proc., vol.230, p. 91, 1992; D. S. Grummon and T. J. Pence, xe2x80x9cThermotractive titanium-nickel thin films for microelectromechanical systems and active compositesxe2x80x9d, Mat. Res. Soc. Symp. Pro., Vol. 459, p. 331, 1997; Q. Su, S. Z. Hua and M. Wuttig, xe2x80x9cMartensitic transformation in NiTi filmsxe2x80x9d, J. of Alloys and Compound, vol. 211, p.460, 1994; S. Miyazaki, et.al., xe2x80x9cShape memory characteristics of sputter-deposited Tixe2x80x94Ni base thin filmsxe2x80x9d, SPIE, vol. 2441, p. 156, 1995; and A. Ishida, A. Takei, M. Sato and S. Miyazaki, xe2x80x9cShape memory behavior of Tixe2x80x94Ni thin films annealed at various temperaturesxe2x80x9d, Mat. Res. Soc. Symp. Proc., vol.360, p. 381, 1995. 11-15]. Few researchers developed actual micro-devices.
The TiNi Alloy Co. has a working microvalve it markets, which closes using a bias mass and opens when the thin film NiTi ligaments are heated [C. A. Ray, C. L. Sloan, A. D. Johnson, J. D. Busch, B. R. Petty: Mat. Res. Soc. Symp. Proc. 276, 161 (1992)]. Krulevitch et al. fabricated a 900 m long, 380 m wide, and 200 m tall microgripper from 5 m thick NiTixe2x80x94Cu film, as well as a functioning microvalve [P. Krulevitch, et al, supra]. Benard et al. fabricated a micro-pump from NiTi film using two designs: polyimide as the biased actuator in one and a complementary NiTi actuator in the other [W. L. Benard, H. Kahn, A. H. Heuer and M. A. Huff, xe2x80x9cThin film shape memory alloy actuated micropumpsxe2x80x9d, J. of Microelectromechanical Systems, vol.7, no. 2, 1998]. Kuribayashi et al. used TiNi films to actuate a microrobotic manipulator [K. Kuribayashi, S. Shimizu, T. Nishinohara and T. Taniguchi, xe2x80x9cTrial fabrication of micron sized arm using reversible TiNi alloy thin film actuatorsxe2x80x9d, Proceedings International Conf. On Intel. Robots and Sys., Yokohama, Japan, p. 1697, 1993]. While the potential applications for SMA MEMS are large, the difficulties with fabricating quality material and achieving the two-way effect is preventing wide spread use of this actuator material.
NiTi films with transformation temperatures above room temperature are difficult to manufacture. Sputtering directly from a 50/50 atm % NiTi target results in films with dramatically lowered transformation temperatures, prohibiting its use as an actuator [J. D. Busch, et. al., supra]. This is caused by the fact that NiTi alloys are strongly dependent on composition, annealing temperatures, aging time, and sputtering parameters [S. Miyazaki, et.al., xe2x80x9cEffect of heat treatment on deformation behavior associated with R-phase and martensitic transformations in Tixe2x80x94Ni thin filmsxe2x80x9d, Trans. Mat. Res. Soc. Jpn., Vol. 18B, pp1041, 1994; A. Ishida, M. Sato, A. Takei and S. Miyazaki, xe2x80x9cEffect of heat treatment on shape memory behavior of Ti-rich Tixe2x80x94Ni thin filmsxe2x80x9d, Materials Transactions, JIM, vol. 36, p. 1349, 1995; and A. Peter Jardine, xe2x80x9cDeposition parameters for sputter-deposited thin film TiNixe2x80x9d, Mat Res. Soc. Symp. Proc., Vol 360, p. 293, 1995]. Of these factors, alloy composition is the most critical.
NiTi alloys and other shape memory alloys are strongly dependent on composition, annealing temperatures, aging time, and sputtering parameters. Composition is the most critical sputtering parameter. Typically, small changes in composition occur during sputtering because titanium readily reacts with other materials. FIG. 1 shows the dependence of transformation temperature on Nixe2x80x94Ti stoichiometry, a shift in composition of as little as 1 atm % can alter transformation temperatures by 100xc2x0 C. [T. W. Duerig, K. N. Melton, D. Stockel and C. M. Wayman, Engineering Aspects of Shape Memory Alloys, 1990]. Titanium is typically used to getter materials, and is often used in vacuum systems to pull down a vacuum by reacting with the gases and condensing. Miyazaki, et al., compensated for the titanium loss by placing titanium plates on top of the alloy target, thereby effectively altering the composition of the target [S. Miyazaki and K. Nomura, xe2x80x9cDevelopment of perfect shape memory effect in sputter-deposited Tixe2x80x94Ni thin filmsxe2x80x9d, Proceedings IEEE Microelectro Mechanical Sys., p. 176, 1994]. Wolf et al. similarly compensated with titanium foils [R. H. Wolf and A. H. Heuer, xe2x80x9cTiNi (Shape Memory) Films on Silicon for MEMS Applicationsxe2x80x9d, J. of Microelectromechanical Sys., vol.4, no.4, p.206, 1995], and A. Gyobu et al. also recently sputtered from a 50/50 NiTi target using titanium compensation [A. Gyobu, Y. Kawamura, H. Horikawa, and T. Saburi, xe2x80x9cMartensitic transformations in sputterdeposited shape memory Tixe2x80x94Ni filmsxe2x80x9d, Mat. Trans. JIM, vol. 37, no. 1-6, p.697, 1996]. The other method of compensating for the titanium loss is to use a multigun co-sputtering system. For example, Krulevitch et al. used a DC magnetron system to sputter from individually powered Ni, Ti, and Cu targets [P. Krulevitch, A. P. Lee, P. B. Ramsey, et.al., xe2x80x9cThin film shape memory alloy microactuatorsxe2x80x9d, J. of Microelectromechanical Sys., vol.5, no.4, 1996].
A further complication is that the NiTi phase is very narrow at low temperatures. Slight shifts in the Ni:Ti stoichiometry can cause precipitate formation, and complicate the metallurgical heat treatment required to establish a desired transformation temperature. It would be advantageous to develop a simple approach that could produce a deposited film with composition similar to the target.
Thin film NiTi fabricated by sputtering offers a promising new material for solid state actuation in the MEMS field as well as new possibilities for medical devices, because of its large energy density(1 J/g) and large displacement (10% strain). Since NiTi SMA shape memory alloys are heat actuated, improved performance can be achieved at microscales. Frequencies of several hundred hertz can be achieved [J. Favalukis, A. S. Lavine, G. P. Carman: Proc. SPIE 3668, 617 (1999)]. Specifically, with a smaller mass and larger surface to volume ratio, heat transfer is substantially increased, power requirements are lowered, and large stresses and strains are achievable. These advantages make NiTi SMA a very promising actuation mechanism fort microdevices.
Sputtering of NiTi thin film from a 50/50 atm % NiTi target produces films with transformation temperatures different from the target due to loss of titanium during sputtering. NiTi films with transformation temperatures above room temperature are difficult to manufacture. Sputtering processes typically produce films with reduced transformation temperatures (i.e. below room temperature), requiring artificial cooling to use as an actuator. Researchers have compensated for this, by placing Ti plates on the target to effectively alter the composition of the target, or to sputter off of a nonstoichiometric NiTi target.
A microscale actuator for active flow control could be implemented using the SME. In recent years the combined evolution of MEMS (microelectro-mechanical systems) technology and active materials has produced advancements that can make Active Flow Control (AFC) practical [C. M. Ho and Y. Tai, xe2x80x9cMems: Science and Technology,xe2x80x9d Application of Microfabrication to Fluid Mechanics, FED V. 197, ASME 1994, pp. 39-49, 1994]. Active Flow Control (AFC) represents an advanced concept for reducing drag, controlling flow separation, improving flight control effectiveness, and manipulation of wake vortex interactions in aircraft systems. The AFC concept has been around for the last 30 years. The obstacle to its successful implementation has been a lack of a compact rugged sensor-actuator technology.
Previously, it was difficult to sputter deposit NiTi films with transformation temperatures above 25xc2x0 C. from a single unmodified 50/50 atm % NiTi target. The present invention achieves transformation temperatures above 25xc2x0 C. from an unmodified 50/50 atm % NiTi target by increasing the temperature of the target during deposition of the NiTi thin film. Furthermore, this process for the production of SME thin films produced a film that exhibited two-way SME without an external bias force.
The present invention is directed to a two-way NiTi thin film shape memory effect device and a method for fabrication of such a device by depositing (a 50/50 atm %) NiTi thin film by DC sputtering. The films have a transformation temperature about the same as the transformation temperature of the target material. One embodiment of the invention involves heating the target to temperatures greater than about 400xc2x0 C. before depositing the NiTi SME thin film on the substrate.
Specifically, this method for fabrication does not require compositional modification of the 50/50 atm % NiTi target. Films that were produced by gradual heating of the target during deposition of the thin film produced a compositionally graded film. The compositional gradation occurs through the film thickness. By gradation, we mean a gradual change in the composition of the sputter deposited material. This gradation produced films exhibiting a two-way SME. The simplicity of this new process can increase the commercial use ofNiTi thin films in microactuator devices by reducing fabrication complexity and costs. The control over the composition by control of the target temperature allows the transition of the martensite to austenite to occur at a temperature above room temperature. Therefore, practical devices can be fabricated that require only heating to cause a shape change. The two-way SME effect means that the device can be repeatedly cycled by heating and cooling, changing shape with each cycle without any external bias force.
These and other features, aspects, and advantages of the present invention will be better understood with regard to the following detailed description and accompanying drawings.